1. Field of the Invention
This invention relates to a sealing stopper or piston for a syringe provided with a backstop function and to a prefilled syringe using the same.
2. Description of the Prior Art
As a method of feeding an injection agent as one agent form of a medicament to a medical scene, there is the system of a prefilled syringe, in which the injection agent is previously prepared (charged) in a syringe-cum-container, transported and stored with the end part thereof sealed by a piston. The front end of the injection barrel is fitted with an injection needle or administration device during administration, and the piston is forcibly thrust toward the front end and slidably moved in the injection barrel so as to cause the injection agent to flow out of the injection needle side, as depicted in JP-B2-3387775. This method has lately been applied to various uses, because of the advantages that (1) operation is very simple, (2) administration can be accomplished with the correct administered quantity and without misuse of a medicament agent even in an emergency and (3) there is no necessity of transferring a liquid medicament, which results in easy avoidance of contamination with micro-organisms, as well as obtaining high sanitary properties. The prefilled syringe is of a disposal type such that the prefilled syringe, once used, should be abandoned due to sanitary problems such as infectious diseases.
Since the above described syringe, once used, is ordinarily abandoned in a usable state if refilled with a liquid medicament, there is a possibility that the abandoned syringe can be reused for a criminal act such as injection of stimulants, or can be misused again in a medical scene. In order to completely prevent these dangers, it is desirable to provide such a mechanism such that once used, the syringe is no longer reusable.
As a syringe provided with such a mechanism, for example, there is a structure in which a plunger threaded with a piston has a reuse-preventive projection which is engaged with a recess of an injection barrel when the plunger is pushed, so that the plunger cannot be withdrawn (called a backstop), as shown in WO 01/64266 A1. However, the mechanism of this syringe is so complicated that its production cost is considerably increased.